This invention relates to a magnetic heating device, for example for use in heat treatment of metal components.
The use of alternating magnetic fields for the heat treatment of metallic articles has been known for many years. Examples of disclosures of such techniques are: U.S. Pat. No. 1,335,453; U.S. Pat. No. 3,965,321; U.S. Pat. No. 4,281,234; U.S. Pat. No. 4,673,781; U.S. Pat. No. 4,761,527; U.S. Pat. No. 4,156,097; EP 0183209; and EP 0459837. All of these disclosures relate to single phase arrangements in which one or two coils are wound around a C-shaped core, with the article to be treated being placed in the gap in the C.
DE-A-277870 discloses an arrangement in which three-phase current is used to supply three coils on three C-shaped magnetic yokes to melt a metal article placed in the common air gaps in the yokes. One of the coils is connected or wound in the opposite direction to the other two.
Greater heating efficiency can be obtained by the use of three phase electrical supply to three coils or pairs of coils around separate cores.
According to the invention there is provided a magnetic heating device comprising three electrical coils, each coil being wound around a respective magnetic yoke having a gap therein, the three yokes being arranged such that the gaps are adjacent to each other or coincident, an article to heated being located. In use, in the gaps in the three yokes, so as to direct the magnetic field generated in each yoke by its respective coil through the article, each coil being connected to a respective different phase of a three phase low frequency alternating electrical supply, and one of the coils being wound in the opposite direction to the other two coils.
By winding one coil in the opposite direction to the other two (or connecting electrically in the reverse directionxe2x80x94the effect is the same), when the magnetic field produced by one phase is at 100%, the other two phases are producing magnetic fields in the same direction and at a significant proportion of the full field, for example at least 50%.
The magnetic fields generated by the three C-shaped yokes are coupled with each other, either by a common polo piece linking the yokes on each side of the air gap, when the yokes are arranged side by side, or by arranging for the magnetic fields to be directed through the same part of the article to be heated, as in the case where the yokes are arranged at 120xc2x0 to each other.
Preferably. three pairs of coils are used, each pair being wound around a respective one of the magnetic yokes, the coils in the pair being connected electrically, either in series or in parallel. Preferably, the coils of each pair are arranged one on each side of the air gap, as close as possible to the air gap.
The three phases may be connected in star or delta formation.
Each coil or pair of coils is either mounted around opposite ends of a single C-shaped core or yoke, preferably made up of thin insulated silicon steel plate, or around three individual C-shaped cores, preferably of the same type. Each pair of coils must be connected so that the current in one flows in the opposite direction to the current in the other coil. One pair of coils must be connected so that the windings are connected in the opposite direction to the other two pairs of coils.
The C-shaped cores or yokes are preferably constructed so that the air gap may be varied. This may be achieved by slidably mounting the upper part of the C with, for example, a hydraulic ram or the like to raise and lower the upper part to change the size of the air gap. In this way, the magnetic flux can be concentrated as efficiently as possible in the article to be heated.
As each electrical supply phase voltage fluctuates between positive and negative voltages, each pair of coils creates a magnetic flux which passes around the core and through the air gap. The polarity of the magnetic field in the air gap oscillates. If a ferrous or non-ferrous metal part is placed in the air gap, the following will happen.
1. In the case of a ferrous part, the magnetic domains within the material will be stressed in fluctuating directions, due to magnetic attraction, creating hysteresis loss within the metal, which will rapidly heat up uniformly;
2. In the case of most non-ferrous metals, the crystalline structures will be stressed by fluctuating magnetic repulsion which again will cause the metal to heat up.
Preferably, the coils or pairs of coils, where separate cores or yokes are used, will be arranged side by side along the article to be heated, but may be arranged radially around a cylindrical or tubular article with the axes through the gaps spaced by 120xc2x0. In this case, the apparatus may be used for stress-relieving welds in tubes.